Details should remove margin-bottom on the last child

[NickColley]

<details class="govuk-details" role="group" open="">
  <summary class="govuk-details__summary" role="button" aria-controls="details-content-0" aria-expanded="true">
    <span class="govuk-details__summary-text">
      I don't know the local council
    </span>
  </summary>
  <div class="govuk-details__text" id="details-content-0" aria-hidden="false">
    <div class="govuk-form-group">
      <label class="govuk-label" for="postcode">
        Enter your postcode
        <span class="govuk-hint">For example, SW1A 2AA</span>
      </label>
      <input class="govuk-input govuk-input--width-10" id="postcode" name="postcode" type="text">
    </div>                 
  </div>
</details>

.govuk-details__text > :last-child {
    margin-bottom: 0;
}